Résumé : The pathophysiology of ovarian hyperstimulation syndrome (OHSS) remains unclear. Several lines of evidence indicate that OHSS is associated with a stimulation of the renin-angiotensin system (RAS), but its functional significance as well as its role in the pathogenesis of the syndrome are not yet determined. OHSS is associated with high plasma and ascitic concentrations of total renin, renin activity (RA) and angiotensin II (Ang II). Their ovarian or renal origin is, however, still a matter of debate. To clarify these issues further, total renin, active renin, prorenin, RA and aldosterone were measured in plasma and ascites of nine patients who developed severe OHSS after in-vitro fertilization. Blood and ascites were sampled simultaneously during therapeutic paracentesis. Total renin and prorenin concentrations were significantly higher in the ascites (mean concentration +/- SE respectively of 5920 +/- 1430 mIU/l and 5250 +/- 1350 mIU/l) than in the plasma (respectively 3060 +/- 740 mIU/l and 2000 +/- 460 mIU/l) (P = 0.020 and 0.017 respectively). Conversely, active renin and RA concentrations tended to be lower, although not statistically significantly so in the ascites (respectively 670 +/- 190 mIU/l and 47 +/- 11 ng Ang I/ml/h) than in the plasma (respectively 1060 +/- 370 mIU/l and 75 +/- 21 ng Ang I/ml/h). Aldosterone concentrations were significantly higher in the serum (2609 +/- 374 pg/ml) than in the ascites (2025 +/- 347 pg/ml) (P = 0.015). The concentration gradient between plasma and ascites for total renin and prorenin supports the hypothesis of their ovarian origin in ascites and, to a large extent, in plasma, while it is likely that the high plasma active renin and RA concentrations reflect a peripheral activation of the RAS. In conclusion, the present findings are consistent with a marked stimulation of both ovarian and renal RAS during OHSS.